Ultrasonic non-destructive examination and structural health monitoring of materials produces many challenges for individuals involved with the examination or monitoring. Two types of inspection conducted on materials include bulk wave investigation and guided wave investigation. Each of the investigation techniques has its own advantages and disadvantages.
Guided wave investigation, in particular, evaluates materials by exposing those materials to guided waves, waves that propagate internally between or along boundaries; however, conventional systems and methods employed to produce guided waves have significant shortcomings. Unique dispersion curves can be calculated for every structure of interest. Dispersion curves exemplify all of the mode possibilities in that structure. Each point on the curve exhibits different displacement characteristics that may be utilized for different inspection needs. It is desirable to design special sensors that are capable of efficiently exciting different regions on these curves curve. It would also be advantageous to have special sensors that are capable of exciting multiple locations or sweeping throughout the dispersion curve space to take advantage of the characteristics of multiple locations on the curve.
There is a need to provide a method for introducing a guided wave into a material, such as a rail, as the sensor is moved along the structure.
There is a further need to provide a method and apparatus to allow for changing the effective incidence angle of guided waves as they are inserted into material to be evaluated, thereby exciting multiple modes and allowing scanning throughout the volume of the structure. There is a further need to provide specialized sensors for structural health monitoring, which are capable of exciting dominant shear horizontal displacement. Shear horizontal displacements are desired for numerous reasons. For example, shear horizontal waves are less affected by liquid loadings than conventional longitudinal guided waves. Also, shear horizontal waves, in some cases, can provide increased penetration power and improved damage detection sensitivity.
There is also a need to provide a method and apparatus for investigation of rail heads, which minimizes the need for bulk wave investigation and instead uses guided waves for inspection, thereby overcoming the deficiencies of bulk wave investigations.
There is a further need to provide a guided wave inspection technique that will investigate rail heads, webs, and bases and prevent anomalies obtained during non-destructive examination from field factors such as rail shelling. Additionally, it may be advantageous to have a sensor that is used to detect shelling.